The present application relates to an elongated implant device and bone stabilization device.
Bone anchors which are respectively inserted into adjacent human vertebrae may be connected to the implant device, in particular to the slidable sleeve and the first end of the rod-shaped member in order to stabilize the vertebral column. Since the sleeve is arranged to be slidable with respect to the first end of the rod-shaped member, some degree of flexion or torsion between vertebrae of the vertebral column can be maintained. Moreover, the dampening element may absorb shocks and relieves and/or distributes the load acting on the vertebrae participating in the bone stabilization.
An example of such a device is known from US 2007/0276,380 A1. A connection unit of an implant device has a first end and a second end. A flexible element extends between the first and the second end, where a cap is attached. A spacer extends between the first end and the cap at the second end. The spacer has a central ring element and, on either side thereof, a resilient spring element. Thereby, the spacer is formed in a sleeve-like fashion such that the flexible element is lead through the spacer, which is thus guided by flexible element. A bone anchor may then each be connected to the first end and the ring element, such that—by means of both resilient spring elements—the ring element may be displaced from a rest position along the longitudinal axis of the flexible element, when forces are exerted on respective bone anchors.
The major forces act in tension or compression along the rod axis. In addition there are secondary loading components, which are bending, shearing and torsional loads. The inner flexible element is mostly acting against such secondary loads. This results in a major bending zone in between the two bone anchors. Such bending is directly transferred onto the flexible spring element, which is arranged between the ring element and the first end.
While the above described flexion does not imply problems with regard to the durability of the resilient spring element arranged between the bone anchors, which may also be valid in the case of torsion, a distinct perspective has to be attained with regard to shear, when material long-term fatigue after several millions of cycles is key.
Therefore, material fatigue of dampening elements, such as resilient spring elements, which are mainly intended to serve for dampening or absorbing forces acting in the longitudinal direction of an implant device, due, e.g., shear stress occurring between adjacent bone anchors should be prevented.
Further, mutual distances between adjacent bone anchors in an implant should be kept small and the resulting degree of flexion of the flexible element should be kept in a limited range.